1. Field of the Invention
The present invention relates to a milling spindle of a machine tool, and more specifically, to a milling spindle of a machine tool having an improved drain (discharge) structure for draining (discharging) the cutting fluid leaked in the milling spindle.
2. Description of the Related Art
FIGS. 6 through 9 illustrate a general structure of a milling spindle of a machine tool.
FIG. 6 is a cross-sectional view showing the structure of a milling spindle, and FIG. 7 is an enlarged cross-sectional view showing the relevant portion thereof.
A milling spindle denoted as a whole by reference number 1 includes a spindle 20 supported within a housing 10 via front-side bearings 12 and 14 and a rear-side bearing 16. A drawbar 24 for drawing a tool into the spindle or pushing a tool out of the spindle is disposed in the spindle 20, and at the center of the drawbar 24 is formed a through coolant passage 22 for supplying cutting fluid to the tool.
A rotor 30 of a motor is attached to an outer circumference portion of the spindle 20, defining a motor for directly driving the spindle with a stator 32.
A rotation-side joint 40 is attached to a rear end portion of the drawbar 24, which constitutes a rotary joint with a joint 50 fixed to the housing 10.
The cutting fluid supplied from a passage 52 of the fixed-side joint 50 is sent via a passage 42 of the rotation-side joint 40 to the through coolant passage 22. A fine gap G1 is formed between the fixed-side joint 50 and the rotation-side joint 40, and a small amount of cutting fluid is leaked through this gap G1.
Due to the structure of the rotary joint, it is difficult to prevent such leakage of cutting fluid completely.
Therefore, a structure is adopted in which a drain hole 60 communicating a drain chamber 62 formed within the housing 10 with the exterior of the housing is provided, so as to discharge the leaked cutting fluid to the exterior of the milling spindle.
The entry of cutting fluid to the bearing 16 or the motor unit 30 and 32 damages the drive unit composed of the bearing and motor and causes serious defects, so that a labyrinth device 70 is disposed to prevent the cutting fluid from entering the drive unit.
However, as shown in FIG. 8, if by some cause a large amount of cutting fluid C1 leaks from the rotary joint portion into the drain chamber 62, the cutting fluid may not be sufficiently discharged through the drain hole 60, and a portion of the cutting fluid C1 may pass through the labyrinth device 70 and damage the bearing 16 or the motor 30 and 32.
As shown in FIG. 9, if the drain hole 60a is widened to have a large diameter, there is a drawback that the drain hole may cause foreign substances D1 such as swarf to enter from the exterior.
Japanese patent application laid-open publication No. 2002-321138 (patent document 1) discloses one example of a mechanism for preventing the cutting fluid leaked from the rotary joint portion of the spindle from entering the drive unit.